1. Field of the Invention
The present invention relates to a fall detection device that can be used for a fall protection of mobile electronic devices and the like, on which a hard magnetic disk drive is installed.
2. Description of the Related Art
Mobile electronic devices inherently have high risks that the devices might be dropped by accident. The mobile electronic devices, such as a notebook PC and some kinds of digital music players, encompassing a hard magnetic disk drive are particularly delicate to impacts and need measures for protection against a fall. When those devices are held still on the earth, gravity is applied on them. But, since acceleration relative to a device is not applied on an acceleration sensor installed on the device during free fall, the acceleration measured by the acceleration sensor becomes zero ideally or extremely small. So, it is usual that the free fall of the devices is detected through a judgment whether or not a measured acceleration is less than a certain threshold, and that a protective counter-measure is taken on the detection result. Such fall judgments are described in Japanese Laid-open Patent 2000-241442 and U.S. Pat. No. 5,982,573. In the Japanese patent document, when an installed three-axis acceleration sensor detects each of the axis components of an acceleration signal that is a small amount of almost zero and continues a certain time period, it is judged that the sensor is in free fall, and a magnetic head is moved to a shelter region to avoid a magnetic drive from breaking down due to impact on the end of falling. In the US patent document, a fall is detected when a magnitude of a compound vector of three-axis accelerations is less than a certain threshold for longer than 90 milli-seconds to move a magnetic head to a shelter region and avoid a magnetic disk drive from breaking down by impact on the end of falling.
In the fall detection described in the Japanese patent document, each of the absolute values of the axis-direction components of acceleration measured by the three-axis acceleration sensor is compared with a certain value (called a threshold value Th), and when each of the absolute values of the acceleration components is less than the threshold value Th, it is judged in free fall. That is, each of the absolute values of the x-axis component Ax, y-axis component Ay and z-axis component Az of the acceleration is compared with the threshold value Th, and it is judged in free fall when each of them is less than the threshold value Th. So, presuming that the measured acceleration A (Ax, Ay, Az) is applied at the origin O in the orthogonal coordinates shown in FIG. 5A, it is judged whether or not the acceleration A is encompassed in a cube having each side length Th in the x-axis, y-axis and z-axis directions. If the acceleration A is composed of only one of the x-axis component, the y-axis component and the z-axis component, it is not judged in free fall when the magnitude of the acceleration is more than the threshold value Th. But, in the cube of a side length Th shown in FIG. 5A, the diagonal direction distance from the origin O to a vertex closest to an observer of the cube is √3 Th. When a measured acceleration A is in the diagonal direction, it is necessary that the magnitude of the acceleration is not less than √3Th in order to avoid a judgment that it is in free fall. There are, however, some cases that it is judged to be freely falling, although the detected acceleration is more than the threshold value Th.
In the fall detection described in the US patent document, the magnitude of the compound vector of the axis-direction acceleration components measured by a three-axis acceleration sensor is compared with a certain value (a threshold value Th) and, when the magnitude of the compound vector is less than the threshold value, it is judged in free fall. In the orthogonal coordinates shown in FIG. 5B, at which origin O the measured acceleration A (Ax, Ay, Az) is applied, it is judged whether or not the acceleration A is contained in a sphere having a center at the origin O and a radius Th. In whichever direction on the sphere the acceleration A is directed, the magnitude of the acceleration A can be compared with the threshold value Th. In order to compare the magnitude of the acceleration A with the threshold value, however, it is necessary to square each of the axis components of the acceleration and to calculate a square root of the sum of the squares. Since a sum square calculation needs a comparatively large-scaled calculation circuit, it would be an obstacle to miniaturize of an IC for the calculation circuit.